1. Field of the Invention
This invention relates to improvements in circuitry for driving polyphase motors, and more particularly to improvements in circuitry for driving polyphase motors in a PWM mode in which the power dissipation requirements of the circuitry is reduced.
2. Description of the Prior Art
Pulse Width Modulation (PWM) techniques, often used to reduce the power dissipation in polyphase motors, chop the current in the coils of the motor at their peak current levels, to achieve maximum torque, to allow rapid accelerations, and to reduce the power dissipated in the chip to a level proportional to the duty cycle. In the past, during this Chopping, the transistors which control the current to the currently active coils of the motor, are switched on and off in a network of switching transistors. During the time the current is switched off, the current built up in the coils is allowed to be dissipated through fly back diodes in parallel with the transistors associated with the active coils. Nevertheless, a voltage drop still exists across the fly back diodes, as the coil current flows through them to dissipate power in the form of heat. This heat, of course, creates a problem which needs to be controlled.